Oh, in that case, I guess I was not talking nonsnese last night
- with no earthing of the neutral at the transformer, I can't see that the supply could possible be called TN-C-S!Kind Regards, John.
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Earthing Arrangement
- Thread starter crozzer99
- Start date
- with no earthing of the neutral at the transformer, I can't see that the supply could possible be called TN-C-S!Kind Regards, John.
P
Paul_C
First letter - Is the neutral of the supply system earthed? Yes, by the connection to an earth electrode at the service entrance, so T.
Second letter - Is the earthing system of the installation connected to the supply neutral? Yes, so that's the N.
Finally - Is the fault-current path caused by an earth fault on the installation separate from the neutral or combined with it? Within the installation it's separate, but due to the E-N bond at the service entrance, it's then combined with the distribution neutral from the supply transformer. So there's the C-S.
So it's not P.M.E. because there's only one earthing point, but it's still TN-C-S by the latter's definition.
Well, per than definition you give (the question at the start of the above quote), everything you go on to say is true.
However, that's not what I thought the '-C' meant. I thought it required that (as in conventional TN-C-S supplies), the return path to the transformer (neutral) and the path from installation to earth shared a single ('combined') conductor for at least part of the run from the transformer end. By that definition, what we're talking about would be TN-S, not TN-C-S, since there is no path from installation to earth along any part of the neutral supply cable.
We may need a referree!
Kind Regards, John.
We'll probably end up in a hamster wheel!
What is agreed is it not PME.
The section of neutral from the transformer to the point at which it earthed is the same for any type of DNO or private system I have ever seen be it TNS or TNCS. If it were in a DNO fuse board and the cables leaving it were PILCs it would generally be TNS.
Dont forget that this earth is a combined metalwork earth AND the starpoint connection to earth
What is agreed is it not PME.
The section of neutral from the transformer to the point at which it earthed is the same for any type of DNO or private system I have ever seen be it TNS or TNCS. If it were in a DNO fuse board and the cables leaving it were PILCs it would generally be TNS.
Dont forget that this earth is a combined metalwork earth AND the starpoint connection to earth
Indeed. PME clearly needs an 'M'!
I'm not sure that resolves the question, does it? My understanding has hitherto been that for it to be -C (i.e. TN-C-S) there has to be at least some length of a single conductor, after the joining of the neutral and earth at the DNO end, which represents both a return to neutral and a potential path to earth when viewed from the consumer's end. Is that right or wrong?
Kind Regards, John.
P
Paul_C
I'd always taken it that the C/S part of TN-S or TN-C-S refers to the overall path of the fault current between the installation and the supply transformer.
With TN-S in the conventional form installed in older built-up districts, a L-E fault on the installation results in the fault current from the transformer going from phase via the installation's earthing conductors, back along the cable sheath to the sub-station, where it will then return to the neutral of the transformer by way of the bond at that point. At no point does the fault current flow along the distribution neutral, hence TN-S.
With the P.N.B. arrangement described above, however, the fault current path is from phase to installation's earthing conductors, then by way of the N-E bond at the service intake to the supply neutral and back to the transformer. The neutral from transformer to installation thus carries both normal operating current and any fault-current caused by an L-E fault on the installation.
Ignoring small currents due to capacitive effects, leakage from damp on the transformer bushings, etc., you could disconnect the earth electrode and the fault current would still take exactly the same path, returning to the transformer by way of the installation's "earthing" system (no longer a true earth, of course, as everything would be floating), via the connection to the supply neutral at the service entrance, then back to the neutral point at the transformer.
With TN-S in the conventional form installed in older built-up districts, a L-E fault on the installation results in the fault current from the transformer going from phase via the installation's earthing conductors, back along the cable sheath to the sub-station, where it will then return to the neutral of the transformer by way of the bond at that point. At no point does the fault current flow along the distribution neutral, hence TN-S.
With the P.N.B. arrangement described above, however, the fault current path is from phase to installation's earthing conductors, then by way of the N-E bond at the service intake to the supply neutral and back to the transformer. The neutral from transformer to installation thus carries both normal operating current and any fault-current caused by an L-E fault on the installation.
Ignoring small currents due to capacitive effects, leakage from damp on the transformer bushings, etc., you could disconnect the earth electrode and the fault current would still take exactly the same path, returning to the transformer by way of the installation's "earthing" system (no longer a true earth, of course, as everything would be floating), via the connection to the supply neutral at the service entrance, then back to the neutral point at the transformer.
I suppose that 'functional' definition probably makes more sense than the more 'structural' one I've been brought up on - and I see that Part 2 of the regs defines TN-C-S in terms of 'combining neutral and protective functions' in one conductor. Of course (and this is probably why I haven't had to have this discussion before) both of the definitions will usually give the same answer for straightforward conventional TN-S and TN-C-S systems; it's only with this 'PNB' system that the two approaches seem to give different answers.
Having said all that, if I understand him correctly, westie seems to agree with my original suggestion, that PNB is TN-S, rather than TN-C-S, so there are still some uncertainties!
Kind Regards, John.
P
Paul_C
I've never really considered the modern TT/TN-S/TN-C-S designations to be especially good anyway, as they don't really convey the full picture of what's happening with regard to earthing systems (as this discussion demonstrates), but have become increasingly used as though they do.
PNB is nor PME, nor are TNCS bonding requirements needed. It is a special case only for use in rural areas with overhead lines.
The ground mounted case discussed is not PNB or PME, in our terms it is an SNE supply (split neutral earth)
The ground mounted case discussed is not PNB or PME, in our terms it is an SNE supply (split neutral earth)
That's somewhat wishful thinking isn't it, given the likely Ze? ... but I guess it depends upon what the 'elevated' is relative to!
Kind Regards, John.
Given that this system has been in use for at least 50 years that I know of and we don't have any problems with it I would suggest it is more fact than thinking.
The document we use was originally written in 1997 and I'm rather sure that better qualified folk than me were involved. Bear in mind that ESCQR has a specific reg about this that would have undergone a huge amount of scrutiny.
H
holmslaw
..
Not in every case I'm afraid, in the 60's and early 70's this was common where we owned the transformer
I got a phone call on Thursday evening from a colleague who had been called out to a local factory. Private system (12 substations) where we operate and control the HV system under contract. The LV switchgear at one substation had been changed a few years ago, along with the transformer LV cables.
They were getting strange voltage and unrecordable earth loop impedances. turns out the N-E link had never been connected!
They were getting strange voltage and unrecordable earth loop impedances. turns out the N-E link had never been connected!
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